Re: NEC-LIST: cross field antennas

Hi,

        This sure is generating alot of controversy. For a long time I
have wanted to build a steerable array of radiating elements for HF
and thus the CFA seemed to be a candidate ( if it would work :-) ). I
looked at the original Hately article in WW and some subsequent work
and was struck by the shall I say 'backyard' approach to the claims
made.

I built a model based on the original design, however the matching
circuit described lacked some finesse ( elegance ? ). After many hours
of bending and cutting of aluminium sheet ( roof flashing is fantastic
for quick and dirty prototypes ) I carefully reviewed the literature
and whilst accepting on face value that the basic premise proposed by
Hately et al could have some merit decided that the design proposed
was not optimum ( the messy quadrature matching rang some alarm bells
).

It is not my intention to compete with 1/4 wave monopoles, they work
fine, are simple and in full conformance to the KISS principle. Just
as the guys who invented lasers did not have flashlights in mind at
the time. I wanted a small compact radiator with broadband / ground
independant properties .... a perfectly radiating dummy load .... an
RF 'lightbulb'.

I built a CFA with much simplified matching and the quadrature phasing
is ( I think ) now done by the structure of the antenna itself
... hence my interest in NEC ... if I could model the structure of
the antenna I could find out if this was the case.

Whilst I am having some difficulty maintaining a broadband feed near /
at 377 ohms the antenna does seem to radiate the power fed to it and
reception is good. I have had problems with losses in previous
attempts at the matching transformer ( thermal imaging cameras are
absolutely fantastic at spotting R2 losses in antennas / ccts ) the
current unit was salvaged from an old linear amp but bandwidth might
be a problem.

I use the 10 meter ham band for my experiments. The antenna is 60 cm
high by 60 cm wide. Input power varies from 1 to 100 watts. Elevation
is from 0.5 meter ( wooden chair ) to 10 meters ( 70 mm PVC water pipe
). SWR is less than 1.3 across the entire band.

Regards.

----------
From: Grant Bingeman <DrBingo_at_compuserve.com>
To: paul moody <paulmoody_at_onaustralia.com.au>
Subject: Re: NEC-LIST: cross field antennas
Date: Friday, February 12, 1999 8:36 AM

Paul, did you make any comparitive field intensity measurements to see
how much lower the signal from the CFA was relative to a conventional
quarter-wave monopole operated over a radial-wire ground system for a
given power input? There is a procedure for this in FCC 73.186, and
if this were done at 1800 kHz with professional, calibrated test
equipment and engineers familiar with same, it would help to put the
CFA question to bed. To my knowledge, this simple comparitive test
has never been performed in a scientific and rigorous manner.

Are you using low-loss ferrite in your transformer? What is the
lowest frequency you operated your modified CFA? Shall we call it the
"Moody Antenna?"

Hey, if the CFA works, that would be great. I just don't think you
will be able to get much signal out of it.

Grant Bingeman, P.E.
----------
Received on Fri Feb 12 1999 - 08:05:15 EST